1. Field
Embodiments of the present invention relate to rolling mill stand fluid lubricated bearings supporting roll necks of rotating rollers that are used to roll metals into elongated webs. More particularly, embodiments of the present invention relate to hybrid bearings having hydrostatic fluid bearing pads within hydrodynamic bearing bushings and lubrication systems that provide an isolated source of pressurized lubricant to each respective bearing pad.
2. Description of the Prior Art
FIG. 1 shows a known rolling mill stand bearing assembly 10 that is rotatively supporting a rotating mill roll 12 by a roll neck section 14 and a complimentary rotating bearing sleeve 16. The bearing assembly 10 is supported by a bearing chock 18 that is in turn supported in a mill roll stand (not shown). A bearing bushing 20 is interposed between the roll neck 14/bearing sleeve 16 and the bearing chock 18 and is fixed in the chock. The bearing bushing 20 provides a hydrodynamic bearing surface for full hydrodynamic lubrication (i.e., a self-generated pressurized fluid lubricant film layer between the bearing bushing and the bearing sleeve 16) within roller rotational speed, applied radial load and lubricant viscosity operational design parameters. In the event that any of those operational parameters fall outside of design parameters—for example during mill stand start-up phase or slow speed rolling at high loads to achieve specific meturlugical properties—it is possible that the bearing 10 will not generate a sufficient hydrodynamic fluid film for desired operation. Lack of a fluid film increases potential for undesirable metal to metal contact between the bearing bushing and the roll neck 14/bearing sleeve 16 and potential bearing wear or failure. In the past, one or more hydrostatic bearing pads 30, 40 have been formed in the bearing bushing 20. The hydrostatic bearing pads 30, 40 are coupled to a source of pressurized fluid lubricant 50 in order to generate additional lubricant fluid film between the bearing bushing 20 and the roll neck 14/bearing sleeve 16. Oil film bearings that operate with both hydrodynamic and hydrostatic features are hybrid oil film bearings. Such bearings only function as “pure” hydrostatic bearings at a speed of 0 RPM; at any other speed they function with hybrid characteristics of both hydrostatic and hydrodynamic bearings combined.
Referring to FIGS. 1 and 2, the pressurized lubricant source 50 is fed to each of the respective bearing pads 30, 40 by a common lubricant passage 22 formed in an axial end of the bearing bushing 20 that branches to respective upstream passages 32, 42, flow restrictors 34, 44, and downstream passages 36, 46. The flow restrictors 34, 44 are typically visco-jets that isolate the respective bearing pads 30, 40 from each other in the event of an unexpected fluid film disruption in one of the pads that would facilitate rapid flow of lubricant through the disruption area and potential starvation of lubricant flow to the other otherwise normally functioning bearing pad. Multiple bearing bushings 20 in a mill roll stand share a common pressurized feed manifold 52 coupled to one or more pressurizing pumps 60. The pump 60 receives conditioned fluid lubricant (e.g., cooled and filtered oil of a desired viscosity range) via a pump inlet 62 and discharges higher pressurized oil via pump outlet 64 into the common oil feed manifold 52. Known rolling mill stand bearings are shown and described in U.S. Pat. Nos. 5,000,584 and 6,468,194, the entire contents of each of which is incorporated herein by reference as if fully set forth herein.
As previously described, one or more rolling mill bushings 20 having multiple hydrostatic bearing pads 30, 40 that share a common pressurized lubricant source (e.g., common manifold 52) reduce risk of simultaneously losing oil pressure through a leak in the common lubricant source restricting flow loss out of each bearing pad 30, 40 with a check valve at the junction of feed pipe 50 and bushings passages 42 and 32 (not shown). In the event of an oil pressure drop upstream of a flow restrictor 34, 44 partial pressure is maintained in the respective upstream lubricant passages 32, 42, so that the corresponding bearing pad is capable of maintaining a pressurized fluid film for at least a short time period. Similarly the flow restrictors 34, 44 slow pressurized lubricant flow into a respective bearing pad 30, 40 that has experienced a pressure failure. For example, absent use of the flow restrictors, if hydrostatic bearing pad 30 does not maintain a hydrostatic fluid film and experiences a rapid pressure drop, pressurized lubricant flow rate will increase into that bearing pad, possibly starving pressurized oil supply to the other pad 40 or other bearings that share the common pressurized oil supply.
Flow restrictors 34, 44 by their nature resist and inhibit oil flow therethrough as flow velocity increases, so additional fluid pressure is required to overcome the restrictions. In a typical rolling mill application an additional 3000 psi (20700 KPa) oil pressure must be generated to overcome visco-jet flow restrictors. Additional pressure requirements increase needed system pumping capacities and energy costs associated with operating the pumps.